Free-form optical surface measuring apparatus and method

ABSTRACT

A surface measuring apparatus for measuring a surface shape of an element includes a measurement frame having a mount for mounting the element to be measured, a stage including a rotatable device, the stage being movable in at least a first direction relative to the measurement frame, and a contactless distance measurement device for measuring in the first direction a distance between the measurement frame and a predetermined measurement surface provided on the rotatable device. The apparatus further comprises a second distance measurement device for measuring in a second direction a second distance between the device and a selected position on a surface of an element mounted relative to the measurement frame and a rotation measurement device for measuring an angle of rotation between the first and second direction. In this way, aspheric or free-form surfaces of optical elements can be measured easily in closed loop without introducing abbe-errors.

The invention relates to a surface measuring apparatus for measuring aposition on a surface of an element to be mounted thereon, comprising: ameasurement frame comprising a mount for mounting the element to bemeasured; a stage comprising a rotatable device, the stage being movablein at least a first direction relative to said measurement frame; and acontactless distance measurement device for measuring in said firstdirection a distance between said measurement frame and a predeterminedmeasurement surface provided on said rotatable device.

In U.S. Pat. No. 6,008,901 surface shapes of for instance opticalelements are measured by a position measurement device that is able tomeasure a contour using a reflection signal coming from the contour tobe measured. However, for increasing dimensions and wilder inclinationsof such contours to be measured, the reflection signals tend to bedeflected in various directions. These reflection signals can then be nolonger received by the measurement devices that are used, in particular,the interferometer beams are reflected away from the interferometricdetection, so that a measurement signal is lost and the surfacemeasurement is out of bounds.

Such aspheric elements are an example of elements that can be measuredby the above referenced type of measurement apparatus. In particular,surface contour measurement has become a great issue sincehigh-precision local shaping methods were developed in order tomanufacture (aspheric or free-form) optical surface elements thattheoretically greatly reduce the number of aberrations. As has becomegenerally appreciated, these complex optical elements, when properlydesigned and manufactured, are able to greatly simplify complex(multiple) lens designs and at the same time maintain or even increasethe performance characteristics of such designs when used for highprecision optics.

U.S. Pat. No. 4,575,942 discloses a stage device with a rotatable devicemounted thereon. It is however not used for surface measurementpurposes.

EP512356 and EP398073 disclose a measurement device for measuring asurface. A measurement frame is provided with a movable stage thereon,the stage comprising a measuring device that can be shifted relative tothe stage. This introduces the risks of undetected torsions, which couldinfluence the reliability of the surface measurement.

The invention has as one of its objects to provide a measurementapparatus that is designed to be able to detect a surface contour of anelement with “wilder” inclinations, wherein still form-measurement canbe done with great accuracy up to un positional accuracy.

To this end, the invention provides an apparatus of the type mentionedin the preamble, comprising the features of the annexed claim 1. Inparticular, according to the invention, said predetermined measurementsurface is formed by a surface of said rotatable device.

By providing a rotatable device, specifically, the invention allows acontour scanning distance sensor to be placed generally orthogonal to asurface to be measured, so that a reflecting measurement signal is notlost but can be adequately picked up by the sensor. Further, inparticular, the invention has as a benefit, that it allows a directmeasurement on the rotatable member itself. Through this, there are nopositional errors introduced, for instance via a bearing that rotatablycouples said rotatable member to said stage or via inaccuracies of theplacement of the stage. The inventive apparatus therefore allows for anultra fine nm precision measurement while maintaining flexible asregarding the inclinations in the surface contour to be measured. Inthis respect the apparatus further preferably comprises a seconddistance measurement device, for measuring in a second direction asecond distance between said device and a selected position on a surfaceof an element mounted relative to said measurement frame; and a rotationmeasurement device for measuring an angle of rotation between said firstand second direction. In addition, the apparatus may comprise a thirddistance measurement device for measuring in a third direction a thirddistance between said stage and said measurement frame.

Although such system can be calibrated with respect to a specificrotation angle of the rotatable device, so that the measurement surfaceneed not be perfectly circular, preferably, said measurement surface isrotation invariant. By “rotation invariant” is meant that themeasurement surface remains at least invariant under small discrete orcontinuous rotations. An example of rotation invariant surfaces arepolyedric forms or circular forms. In this way, a distance measurementcan be performed generally independent of the angle of rotation of therotatable member.

Further, preferably, at least said first distance measurement devicecomprises an interferometer and said rotatable member is formed by areflective member that has a perfect spherical or cylindrical shape overat least said measurement surface. An embodiment with a cylindricalshape effectively allows only a rotation over a single rotation axis,oriented preferably perpendicular to said first and second directions. Abenefit of such a single rotation axis is that the degrees of freedom ofthe apparatus are constrained, eliminating the possible rise ofpositional errors etc. It however also restricts the possibilities tomeasure inclinations that are oriented in the direction of saidrotational axis. To this end, in an embodiment where the rotatablemember is rotatable through multiple axis, for instance, where therotatable member comprises a perfectly spherical surface over at least ameasurement surface area, also inclinations in different directions canbe measured.

In a preferred embodiment, said reflective member is comprised in ahousing provided on said stage, and wherein said reflective member iscoupled directly to said second distance measurement device, saidhousing comprising a focusing member for focusing light from said firstdistance measurement device on said reflective member, so that areflective light beam emanates virtually from the central axis of saidreflective member. In this way, the reflection beam is reflected back tointerferometer so that the arrangement is generally insensitive todivergence due to a curved surface of the reflective member and lateraldisplacement thereof. Preferably, said focusing member is a cylindricallens and said reflective member is cylindrical or alternatively, saidfocusing member is a spherical lens and said reflective member isspherical.

In one embodiment, said measurement frame comprises a reflective mirror,and wherein said stage comprises a beam splitting element, wherein abeam path of said first distance measurement interferometer travelsdirectly between said reflective mirror, said beam splitting element andsaid reflective member, wherein said beam splitting element is coupled alight source, said beam splitting element further coupled to aninterferometric light detector. This interferometric configurationallows a interferometric reference beam coupled directly to themeasurement frame, so that the configuration is generally insensitive todisplacements of said stage along the said beam path, when thepositioning of the reflective member remains unmoved.

To allow more degrees of freedom in the above mentioned setup, in apractical embodiment, the said stage may be movable in two orthogonaldirections and said stage comprises a third distance measurement devicefor measuring in a third direction a third distance between said stageand said measurement frame, said third direction being orthogonal tosaid first direction. Furthermore, preferably, the apparatus comprises arotatable mount for mounting an element to be measured. In order todetect tilt of the element relative to said mount, said mount comprisesa reference surface for allowing a measurement relative to saidmeasurement frame.

In the setup of the invention, for wilder inclinations, the reflectionbeam may be divergently reflected, even if use is made from a relativelynarrow beam. To preserve a sufficiently detectable reflected signalpreferably said second distance measurement apparatus comprises aninterferometer comprising an auto focus detector for focusing aninterferometric beam of said interferometer on a selected position onsaid surface of said element. Such an auto focus feature is per se knownfrom the above referenced-publication. However, preferably, said autofocus detector is provided with a focus distance measurement device inorder to measure a focal distance from said auto-focus detector to saidselected position on said surface of said element. In combination withsaid interferometric measurement beam, said focal distance measurementprovides an absolute zero-level plane from which the interferometricmeasurement can be built up. The zero-level is established within apositioning accuracy of the auto focus detector, which may be in theorder of a few tens of nm, wherein the variations relative to saidzero-level are established with a positioning accuracy of theinterferometer detector, which may be in the order of a few nm. It willbe understood that such a focal distance measurement device may be usedindependent of the above mentioned surface contour measurement setup.

Furthermore, preferably, said second distance measurement interferometermay comprise a tilt detector for detecting a level of tilt of saidelement to be measured. Such a tilt detector is per se known from theabove reference publication. Preferably, said tilt detector is arrangedto detect a level of tilt of the element to be measured in a directionorthogonal to said first and second directions. Specifically, asexplained in the afore going, said direction may generally correspondwith an axis of rotation of the rotatable member. Smaller inclinationsmay thus be measured along this axis of rotation, while preserving thebenefits of a single axis of rotation.

The rotation of said rotatable member may be provided independent of ameasured contour surface, for instance in a feed forward loop. This ispossible when the surface to be measured is not particularly wild andgenerally known in contour. In a preferred embodiment however, said tiltdetector is coupled to said stage, so as to position said seconddistance measurement interferometer orthogonally to a measured contourof said element.

The invention further relates to a method for measuring a position of adevice that is rotatable relative to a movable stage, comprising:providing a measurement frame; providing a predetermined measurementsurface on said rotatable device; and measuring directly in a firstdirection a first distance between said measurement frame and saidpredetermined measurement surface provided on said rotatable device.Such a method provides an accurate positioning measurement of saiddevice, generally independent of the angle of rotation and/or ofpositional errors of said stage.

The invention will further be illustrated with reference to the annexeddrawings. In the drawing

FIG. 1 shows a schematic partial view of an embodiment of the apparatusaccording to the invention;

FIG. 2 shows a preferred embodiment having movable stage and a lightguidance mounted on said stage;

FIG. 3 shows an alternative embodiment of the apparatus according to theinvention;

FIG. 4 shows a general setup for the apparatus according to theinvention;

FIG. 5 shows a perspective schematized view of the apparatus accordingto the invention;

FIG. 6 shows a preferred embodiment of a contour scanning distancesensor;

FIG. 7 shows a typical use of the auto focus element in the contourscanning distance sensor of FIG. 6; and

FIG. 8 shows a ray analysis of a preferred interferometric setup for adistance measurement apparatus according to the invention.

In the drawings, the same or corresponding elements will be referencedwith the same reference numerals. Turning to FIG. 1 there is depicted aschematic setup for a measuring apparatus 1 for measuring a position ofa rotatable device 2 on a movable stage 3. In FIG. 1, the rotatabledevice may be any rotatable device, for example shaping tool or a workpiece in a high precision manufacturing device, a calibration device formeasuring machine accuracy, or the like. In the remainder, thedescription will be focusing towards measuring apparatus setups, whereinthe rotatable device comprises a contour scanning distance sensorscanning the contour of an element to be measured; and a rotationmeasurement device for measuring an angle of rotation between said firstand second direction. Such a setup will be further elaborated upon inFIG. 4 and FIG. 5 and may be used as a contour measurement apparatus.

The inventive apparatus 1 comprises a measurement frame 4. Suchmeasurement frame 4 is considered a fixed outside world, and to this endis kept preferably as stationary as possible. Independent of saidmeasurement frame 4, a stage 3 is movable relative to said measurementframe 4. On the stage 3 is provided a rotatable device 2, for instance,a contour scanning distance sensor. In FIG. 1 it is shown that the stage3 is movable in two orthogonal directions (indicated by arrows R and Z).The precise guiding systems of the stage 3 are not shown but will befurther illustrated with reference to the subsequent drawings. Theapparatus comprises two independent distance measurement devices 5, 6for measuring in a orthogonal directions an R-distance and Z-distancerespectively, between said stage 3 and said measurement frame 4. Thedistance measurement devices 5, 6 are preferably heterodyneinterferometers, but also other type of contactless distance meters maybe used, for instance white light interferometers or absolute distanceinterferometers. FIG. 1 shows that the predetermined measurementsurface, relative to which a distance measurement is performed, is areflective measurement surface 7 of said rotatable member 2. In the formof rotatable contour scanning distance sensor, specifically, theinvention allows the sensor 2 to be placed generally orthogonal to asurface to be measured, so that a reflecting measurement signal is notlost but can be adequately picked up by the contour scanning distancesensor 2. In particular, the invention allows a direct measurement onthe rotatable sensor 2 itself. Through this, there are no positionalerrors introduced, for instance via a bearing 8 that rotatably couplessaid rotatable member 2 to said stage 3 or via inaccuracies of theplacement of the stage 3. The inventive apparatus 1 therefore allows foran ultra fine nm precision measurement.

FIG. 1 further shows that the rotatable device 2 has a cylindrical form.In this setup, all movements are in a single plane, and rotations onlyoccur with axis of rotation a direction perpendicular to said plane.Such a setup offers good placement accuracy and stability. Other forms,such as spherical forms, allow for rotations in other directions.Furthermore, with reference to FIG. 3, a regular polyedric form isdiscussed. The rotatable device 2 having reflective surface 7 iscomprised in a (not shown) housing provided on said stage 3. Suchhousing provides a bearing 8, such as an air bearing or the like. For nmprecise positioning measurement of the rotatable device 2 the reflectivesurface 7 forms a part fixed relative to said second device 2, forinstance by forming an integral part with device 2. Furthermore,focusing members 9 are present for focusing light from said firstdistance measurement device on said reflective measurement surface 7, sothat a reflective light beam emanates virtually from the central axis ofsaid rotatable device 2. Such a light beam originates from a heterodynelaser arrangement 10, substantially producing light at two close bywavelengths as is well known in the art.

In the shown embodiment of FIG. 1, these focusing members 9 arecylindrical lenses, which together with the cylindrical reflectivesurface form a lens system that reflects a substantially parallel beamback to interferometric detector 5 and 6 respectively.

As depicted detector 5 is arranged to measure a relative distance in theZ-direction, wherein detector 6 is arranged to measure a relativedistance in the R-direction.

Furthermore, by virtue of the presence of said focusing members 9,displacements of the reflective surface relatively lateral to said beamare cancelled and the position detection remains virtually insensitiveto such displacements as should be. Although the light guiding systemwill be further explained with reference to FIG. 8, for the purpose ofunderstanding FIG. 1 the apparatus according to the invention comprisesan interferometric arrangement with beam splitting elements 11 that movetogether with the stage 3 along a line parallel to an R-axis or a Z-axisrespectively. The beam splitting elements 11 are coupled to the laser 10and to interferometric light detectors 5 and 6 respectively through alight guiding structure, schematically illustrated by dotted lines 12.In this way, beam paths 13, 14 of said interferometric arrangementtravels between respectively reflective mirror 15, 16 forming areference surface on said measurement frame 4, said respective beamsplitting elements 11 and reflective measurement surface 7. It followsthat in the embodiment depicted in FIG. 1, an interferometricarrangement for measuring a R-distance along beam 14 comprises laser 10,beam splitter 11, reflective reference mirror 16, reflective measurementsurface 7 and interferometric detector 6. The interferometricarrangement for measuring a Z-distance along beam 13 between saidmeasurement frame 4 and said reflective measurement surface 7 compriseslaser 10, beam splitting element 11, reflective reference mirror 15,reflective measurement surface 7 and interferometric detector 5. Furtherparticulars of this setup will be described with reference to FIG. 8.

In FIG. 1 the light guiding structures from the laser 10 via respectivebeam splitters 11, to the interferometric detectors 5 and 6, are onlyschematically illustrated by dotted lines 12. In FIG. 2 this is furtherelaborated in a preferred light guiding configuration, as well asparticulars regarding a preferred actuation of the sage 3 becomeapparent. The light guiding configuration comprises a guiding mirror 17and a (non polarizing) beam splitter 18 which distributes the light beamfrom laser 10 to the polarizing beam splitters 11 of respective R and Z(heterodyne) interferometric arrangements. This setup provides that thelaser 10 and detector 5 and 6 respectively are well aligned along theR-axis and that the respective detectors 5 and 6 receive a signalindependent of the R/Z position of the stage 3. In particular beamsplitters 11 can move freely along an R-leg 19 and a Z-leg 20 of thelaser beam respectively, parallel to the R-axis and Z-axis, whilesubstantially maintaining their position in the laser beam. As will beexplained with reference to FIG. 8, this movement introduces nodetection error, since the detection is relatively independent of thedistance of said splitters 11 to the reference mirrors 15 and 16respectively. As to a preferred actuation of stage 3, stage 3 movesrelative to a guiding stage 21 that is only movable in R-direction.Stage 3 moves in Z-direction relative to said guiding stage 21. Saidguiding stage 21 contains a light guiding arrangement in the form ofsplitter 18 and guiding mirror 17. In the R-leg 19, beam splitter 11 ispositioned on said guiding stage 21 to split the laser beam from theR-leg towards the beam path 13. In the Z-leg 20, beam splitter 11 ispositioned on stage 3 and moves along the Z-leg 20, while splitting thelight from the Z-leg towards beam path 14. Since only use is made ofsplitting mirrors and guiding mirrors unlimited numbers, high accuracycan be maintained.

FIG. 3 shows an alternative embodiment of the invention. Here, inrelation to the embodiment of FIG. 2, basically more freedom is allowedin the light guidance design, by use of flexible light guides in theform of fibers 22. These fibers 22 allow the laser 10 and detectors 6and 6 placed on positions relatively independent of the stage-movement.Furthermore, splitters 11 and 18 can now be provided on a single stage3, reducing relative positional errors and facilitating-alignment. Forthe interferometrc setup, the guiding stage 21 is no longer used.Instead of the perfectly circular shape of the cross-section of thereflective member 2, in FIG. 3 a regular polyedric shape is used,allowing a relative flat reflection surface for measurement beams 13 and14. Here the beam is not divergently reflected on the reflectivemeasurement surface 7 but travels directly back to the beam splitters 11and respective reflective mirrors 15 and 16.

FIG. 4 and FIG. 5 show a general schematic view in cross-section and aperspective view of a surface contour measuring apparatus 23. Saidmeasuring apparatus comprises a rotatable spindle 24 for mounting anelement to be measured 25 (schematically referenced by the circular θarrow). The element to be measured 25 may be an optical-element such asa lens or aspheric optical element. In this respect the term asphericindicates an element that may be rotationally symmetric along rotationaxis 26 of the spindle 24 but that deviates from a spherical form alongradial contour indicated by reference 27. However, the aspheric element25 may be not rotationally symmetric as well as will be explained infurther below. Above said rotating spindle 24 a contour scanningdistance sensor 2 is present and is rotatable, schematically indicatedby the rotation angle φ. The distance sensor 2 measures a distance srelative to a selected position on a surface of an element 25 mounted onsaid spindle 24. Schematically indicated by (preferably contactless)measuring/reference elements 28, the position of the spindle 24 ismeasured relative to measurement frame 4. In addition, the radial offsetof the spindle is measured. To also detect tilt of the element 25relative to said spindle 24, said spindle 24 comprises a furtherreference surface (not shown). In this way, the measurement loop isclosed, linking every-position on the element 25 directly to themeasurement frame 4 so that the relative position of the element 25towards said measurement frame 4 is known. As such measuring/referencealignment is known in the art, this will not be further elaborated upon.The angle φ of rotation of the distance sensor 2 is known by a rotationmeasurement device, in FIG. 1-FIG. 3 schematically-indicated byreference 29.

From FIG. 4 follows, that all measured positions are known relative to ameasurement frame 4. By the apparatus 1 according to the invention, thesurface contour of element 25 can be derived without introducing Abbeerrors due to alignment, since one is able to measuring the position ofthe rotatable distance sensor 2 by measuring directly on a measurementsurface 7 provided on said rotatable distance sensor 2. In order tofurther eliminate measurement errors, the apparatus 1 is mounted onshock absorbing pillars 30 providing a relative vibration freemeasurement environment.

FIG. 6 shows a preferred embodiment for the surface contour scanningsensor 2. The sensor comprises a two part arrangement, having an(heterodyne) interferometric part 31 for performing an actually measuredoptical path difference between a reference leg 32 and an measurementleg 33; and an auto focus part 34 as will explained further below.

The measurement leg 33 travels through the autofocus part 34 and isreflected back on a scanning surface 35 that is part of an element to bemeasured. The measured optical path difference from the measurement leg33 relative to the reference leg 32 in the interferometric part 31provides (sub)nanometer precision of the displacement of scanned surface35 to a reference position fixed to interferometric part 31. The core ofthe interferometric arrangement 31 is essentially formed by polarizingbeam splitter 36, which is provided with quarter lambda plates 37 toprovide a desired beam path for the measurement beam 33 and referencebeam 32. The interferometric measurement is essentially performed by anarrangement farmed by the laser 10 (which may be the same or a differentlaser used in the distance measurement arrangement depicted in theprevious figures), interferometric detector 38, polarizing beam splitter36, reference surface 39 and scanned surface 35.

To keep the scanning beam 33 in focus, auto focus arrangement 34 isprovided in the measurement leg 33 for focusing the scanning beam 33 ona selected position on surface 35 of element 25. To this end the autofocus arrangement 34 comprises a (non polarizing) beam splitter 40 thatchannels a portion of the reflected scanning beam 33 towards a detectorunit 41. The detector unit 41 detects a difference in balance and/orposition of a pair of focus spots 42, to measure a tilt and/or in focusof the scanning beam. FIG. 7 shows the effects of such in focus and/ortilt in positions: (1) shows a scanned surface 35 in focus of thescanning beam 33 and having a level position, wherein two detector spots42 are centred; (2) shows the scanned surface 35 out of focus, where thetwo detector spots 42 are still balanced but off-centre; (3) shows thescanned surface 35 tilted; where an imbalance is detected between thedetector spots 42. In FIG. 7, the reflected signal is depicted in dottedlines.

The inclination of the scanned surface 35 may be measured via said tiltdetection. Said inclination may also be measured in more directionsusing generalized versions of this setup having more than two detectorspots 42.

Otherwise, as indicated with reference to FIG. 4, the inclination of thesurface 35 may only be measured in the tangential direction of therotating spindle 24, since the radial inclination is (indirectly)measured using the interferometric device 2. The interferometricdistance sensor 2 having autofocus offers an absolute distancemeasurement in addition to the (relative) interferometric measurement.In other words, using the interferometric detection, a relativedisplacement is measured through a measured optical path difference inreference leg 32 and measurement leg 33. This establishes a measuredcontour that has a non-fixed zero level. To this end, the auto focusarrangement 34 comprises a focus distance measurement device 43 in orderto measure a focal distance from said auto-focus detector the scannedsurface 38 of element 30. This device may be a capacitive distance meteror an inductive distance meter. Alternatively a glass-lineal may be usedto provide an absolute distance relative to the measurement surface7/measurement frame 4. The device 2 as illustrated is therefore able toestablish absolute distance measurement within the accuracy of the autofocus arrangement 34 with focus distance measurement device 43; andrelative distance measurement within the resolution of theinterferometric arrangement 31. It may be clear that the contourscanning sensor as illustrated may be used also in non-rotatableconfigurations.

FIG. 8 shows the relative insensitivity of (small) displacements of abeam splitting element 11 depicted in FIG. 1-FIG. 3. In the figure, beamsplitting element 11 is shifted over a distance indicated by d. The beampath for the unshifted splitting element 11 is depicted in grey line asmeasurement beam M1 and reference beam R1; the beam path for the shiftedsplitting element is depicted in black lines as measurement beam M2 andreference beam R2. The beams R1, M1 (and shifted beams R2, N2respectively) are polarized beams relatively orthogonal to each otherand transmitted by the heterodyne laser 10. The R1, M1 beams interfereafter rotating the polarization in a beat signal, that is slightlyshifted by Doppler effects caused by relative variations between the twobeams. The arrangement is such that the reference beam R1 is transmittedunhindered by the beam splitter 11 towards the detector 5. Measurementbeam M1 is however, due to its orthogonal polarization state relative toR1, reflected by the polarizing beam splitter 11 towards referencemirror 15. Towards and from said reference mirror 15 it passes twice aquarter lambda plate 44 which effectively results in 90° relativerotation of the polarization, so that, on entrance of the polarizingbeam splitter 11, beam M1 has now the same polarization state as R1 andis transmitted unhindered by the beam splitter 11 towards themeasurement surface 7. On its way towards surface 7 it passes again aquarter lambda plate 44, which is also passed on the way back. Suchdouble crossing of plate 44 results in another 90° shift of thepolarization, resulting in reflection by the beam splitter 11 towardsdetector 5. In FIG. 8 it is illustrated how the measurement beam M1 andreference beam R1 are shifted after a small displacement of the beamsplitter. It shows that the optical path difference between thereference beam (R1, R2) and measurement beam (M1, M2) is unaffected bysaid shift resulting in an unaffected measurement of the distance(variations) between the reference mirror 15 and the measurement surface7. It follows that the light guidance arrangement as depicted in FIG.1-FIG. 3 does not add errors in position detection, since the positionof the light guidance elements 11 is relatively unimportant.

Although the invention has been illustrated with reference to certainpreferred embodiments, the invention is not limited thereto but mayenclose variations and modifications without departing from the scope ofthe invention. Such variations are deemed to fall within the scope ofthe annexed claims.

1. A surface measuring apparatus for measuring a position on a surfaceof an element to be mounted thereon, comprising: a measurement framecomprising a mount for mounting the element to be measured; a stagecomprising a rotatable device, the stage being movable in at least afirst direction relative to said measurement frame; and a contactlessdistance measurement device for measuring in said first direction adistance between said measurement frame and a predetermined measurementsurface provided on said rotatable device, said rotatable device furthercomprising: a second distance measurement device, for measuring in asecond direction a second distance between said device and a selectedposition on a surface of an element mounted relative to said measurementframe; and a rotation measurement device for measuring an angle ofrotation between said first and second direction.
 2. Apparatus accordingto claim 1, wherein said measurement surface is rotation invariant. 3.Apparatus according to claim 1, wherein at least said first distancemeasurement device comprises an interferometer and said measurementsurface is formed by a reflective member that has a polyedric orcircular shape over at least said measurement surface.
 4. Apparatusaccording to claim 3, wherein said reflective member is comprised in ahousing provided on said stage, and wherein said reflective member iscoupled directly to said second distance measurement device, saidhousing comprising a focusing member for focusing light from said firstdistance measurement device on said reflective member, so that areflective light beam emanates virtually from the central axis of saidreflective member.
 5. Apparatus according to claim 3, wherein saidfocusing member is a cylindrical lens and said reflective member iscylindrical or wherein said focusing member is a spherical lens and saidreflective member is spherical.
 6. Apparatus according to claim 3,wherein said measurement frame comprises a reflective mirror, andwherein said stage comprises a beam splitting element, wherein a beampath of said first distance measurement interferometer travels directlybetween said reflective mirror, said beam splitting element and saidreflective member, wherein said beam splitting element is coupled alight source, said beam splitting element further coupled to aninterferometric light detector.
 7. Apparatus according to claim 1,wherein said stage is movable in two orthogonal directions and saidstage comprises a third distance measurement device for measuring in athird direction a third distance between said stage and said measurementframe, said third direction being orthogonal to said first direction. 8.Apparatus according to claim 1, further comprising a rotatable mount formounting an element to be measured.
 9. Apparatus according to claim 8,wherein said mount comprises a reference surface for allowing ameasurement relative to said measurement frame.
 10. Apparatus accordingto claim 1, wherein said second distance measurement apparatuscomprises: an interferometric part for providing an interferometricmeasurement beam; a movable focus part for focusing said interferometricbeam on a selected position on said surface of said element; aninterferometric detector for receiving said interferometric beam fromsaid selected position and for measuring a distance between saidinterferometric part and said selected position; a unit forautomatically moving said focus part to an in-focus position; and afocus distance measurement device for measuring a relative positionbetween said focus part and said interferometric part.
 11. Apparatusaccording to claim 10, wherein said focus distance measurement devicecomprises an inductive and/or capacitive distance meter or a glasslineal or the like.
 12. Apparatus according to claim 10, wherein saidfocus distance measurement device is coupled to said interferometricdetector in order to provide an absolute zero-level to aninterferometric measurement performed by said detector.
 13. Apparatusaccording to claim 11, wherein said focus distance measurement devicecomprises a distance meter for measuring a relative distance of theinterferometer relative to the auto focus.
 14. Apparatus according toclaim 10, wherein said second distance measurement interferometercomprises a tilt detector for detecting a level of tilt of said elementto be measured.
 15. Apparatus according to claim 14, wherein said tiltdetector is arranged to detect a level of tilt of the element to bemeasured in a direction orthogonal to said first and second directions.16. Apparatus according to claim 14, wherein said tilt detector iscoupled to said stage, so as to position said second distancemeasurement device orthogonally to a measured contour of said element.17. Method for measuring a position on a surface of an element,comprising: providing a measurement frame; providing a stage movablerelative to the frame and comprising a device that is rotatable relativeto the stage; providing a predetermined measurement surface on saidrotatable device; measuring directly in a first direction a firstdistance between said measurement frame and said predeterminedmeasurement surface provided on said rotatable device; measuring in asecond direction a second distance between said rotatable device and aselected position on a surface of an element mounted relative to saidmeasurement frame; and measuring an angle of rotation between said firstand second direction.
 18. Apparatus according to claim 2, wherein atleast said first distance measurement device comprises an interferometerand said measurement surface is formed by a reflective member that has apolyedric or circular shape over at least said measurement surface. 19.Apparatus according to claim 11, wherein said second distancemeasurement interferometer comprises a tilt detector for detecting alevel of tilt of said element to be measured.
 20. Apparatus according toclaim 15, wherein said tilt detector is coupled to said stage, so as toposition said second distance measurement device orthogonally to ameasured contour of said element.